Class PaymentProtocol

Utility methods and constants for working with
BIP 70 aka the payment protocol. These are low level wrappers around the protocol buffers. If you're implementing
a wallet app, look at PaymentSession for a higher level API that should simplify working with the protocol.

BIP 70 defines a binary, protobuf based protocol that runs directly between sender and receiver of funds. Payment
protocol data does not flow over the Bitcoin P2P network or enter the block chain. It's instead for data that is only
of interest to the parties involved but isn't otherwise needed for consensus.